Linear Predictive Coding (LPC) is a digital sound encoding principle according to which the encoder repeatedly constructs, for each short sequence of input samples, a linear all-pole filter that with a certain excitation signal enables producing a replica of the corresponding input sample sequence. The encoder transmits information representing the filter parameters and the exitation signal to the decoder. Known variations of LPC include but are not limited to transformation coding or code excitation according to what is the selected approach to generating the excitation signal, as well as various selections with respect to whether filter parameters are transmitted directly or in some transformed form. Such variations have no effect to the applicability of the general principle of the present invention.
The selection of input signal bandwidth has great influence to the naturalness of the eventually reproduced sound. A narrow bandwidth of the input signal is advantageous in terms of saving required transmission capacity. Accepting a wider band of input frequencies to encoding would enable reproducing the sound in a more natural way at the receiving end, but simultaneously increases the demand for transmission bandwidth.
FIG. 1 illustrates a band split coding principle that offers possibilities for enhancing the quality of reproduced sound while keeping requirements for transmission bandwidth reasonable. The signal coming from an input signal source 101 is taken through a band split filter 102, which directs a certain lower band of the input signal frequencies to a low band encoder 103 and a corresponding upper band of the input signal frequencies to a high band encoder 104. In the digital encoding of speech the lower band includes frequencies from a lower limit near zero to a few kHz, for example 3.4 kHz or 6.4 kHz. The upper band extends above the lower band to some upper limit, like 8 kHz or 12 kHz. The output signals of the low and high band encoders 103 and 104 are combined for transmission and transmitted through some transmitting channel 105 to a receiving device, where a low band decoder 106 and a high band decoder 107 decode the parts of the transmitted signal coming from the low band encoder 103 and high band encoder 104 respectively. A band reconstruction block 108 combines the outputs of the low and high band decoders 106 and 107, after which the reconstructed signal is taken to a sound reproducing arrangement or corresponding signal sink 109.
In a very basic arrangement the low and high band encoders 103 and 104 operate independently, and selection is applied according to whether the outputs of both of them or only the low band encoder 103 are transmitted. More advanced arrangements utilise some information from the low band encoding and decoding in performing the high band encoding and decoding respectively, which is illustrated as vertical arrows between the appropriate functional blocks in FIG. 1. The principle is generally referred to as bandwith extension, and it works well with input signals like speech, where correlation between the low and high bands is strong. Bandwidth extension is discussed for example in a prior art publication Yasheng Qian, Peter Kabal: “Pseudo-wideband speech reconstruction from telephone speech”, Proc. Biennial Symposium on Communications (Kingston, ON), pp. 524-527, June 2002.
FIG. 2 illustrates a known arrangement for high band encoding, in which an input signal coming from a band split filter is subjected to LPC analysis in block 201. From an associated low band encoder an excitation signal is taken. Due to a different excitation sampling frequency the low band excitation signal is not directly usable in the high band encoder, but this can be corrected by taking it through a resampling block 202, which resamples the low band excitation signal onto a suitable sampling frequency. The LPC parameters from the LPC analyser block 201 and the resampled low band extension signal from the resampling block 202 are directed to an LPC synthesis block 203, which produces a synthesized high band signal. The LPC synthesis function implemented in block 203 is an inverse of the LPC analysis function of block 201, so transmitting the parameters used in the LPC synthesis will enable a receiver (not shown in FIG. 2) to similarly synthesize the high band signal. In order to align the synthesized signal energy with the original high band signal the high band signal gain needs to be calculated in a gain control block 204, which is coupled to receive the original high band audio signal (or at least information about its signal energy) as well as the output of the LPC synthesis block 203. The output of the gain control block 204 is transmitted to the receiver along with the parameters obtained from block 203.
The drawback of the arrangement of FIG. 2 is that in situations where the low band contains a strongly voiced signal but the frequency spectrum of the high band is relatively flat, it causes annoying, unnatural effects to the synthesized audio signal. This effect is rarely encountered with speech, but is clearly noticeable for example when the input signal is music.